The present invention relates to a scanning electron microscope system for measuring surface configuration and other data at high resolution by projecting an electron beam to scan a specimen, by detecting reflected electrons and secondary electrons generated from the specimen and by displaying a scanned image of the specimen based on a detection signal on an image display unit such as a cathode ray tube.
In the past, in a process to manufacture a semiconductor, a scanning electron microscope (SEM) has been used for performing observation and examination of configurations on a circuit pattern or a contact hole prepared on a wafer. In recent years, integration in a semiconductor device have become higher, a circuit pattern to be transferred or size of a contact hole have become increasingly finer, and there are now strong demands on the development of a scanning electron microscope system with high resolution for observing and examining these configurations and patterns. Contact holes with larger aspect ratio or deep graded steps are now formed more frequently on wafers as circuit patterns have become finer, and there are now more requests for measuring these configurations and patterns, not 2-dimensionally but 3-dimensionally.
In general, to perform 3 dimensional measurement, a primary electron beam is projected with tilting onto the specimen and a plurality of tilted observation images are synthesized and measurement is made. For this purpose, there may be a method to tilt a specimen stage or a method to project the primary electron beam with tilted angle onto the specimen without tilting the specimen. In a conventional type scanning electron microscope, when the primary electron beam is tilted, the primary electron beam passes through outside an axis of an objective lens, and abaxial aberration occurs. As a result, resolution is decreased. For this reason, it has been practiced in the past to perform observation on a tilted image by tilting the specimen stage.
In contrast, the present inventors have invented an electron beam optical system for performing observation on a tilted image without tilting the specimen stage. This system is disclosed in JP-A-11-67130.
Referring to FIG. 4, description will be given below on an electron beam optical system disclosed in JP-A-11-67130. FIG. 4 is an essential portion of an electronic optical system 1 of the electron beam optical system, and it is shown by an equivalent optical system.
In FIG. 4, reference numeral 2 denotes an optical axis of an electron beam optical system, 3 is a deflected orbit of a primary electron beam, and 4 is a specimen.
On the optical axis 2, there are arranged a deflector 5 (coil or electrostatic deflector), an objective lens 7, and a compensation deflection coil 8 to be superimposed on a lens magnetic field of the objective lens 7. In order to compensate abaxial aberration of the objective lens 7, which serves as a magnetic field type lens, the compensation deflection coil 8 is provided. A decelerating voltage is applied on the specimen 4, and a decelerating electrostatic lens 12 is formed by the deceleration field. To compensate abaxial aberration of the decelerating electrostatic lens 12, a compensation deflection electrode 9 is provided. By superimposing the magnetic field type objective lens 7 (compensated by the compensation deflection coil 8) on the decelerating electrostatic lens 12 (compensated by the compensation deflection electrode 9), an electrostatic magnetic field complex objective lens (superimposed lens) 13 is formed.
In the electron beam optical system as shown in FIG. 4, the primary electron beam is deflected by the deflector 5 and the electrostatic magnetic field complex objective lens 13 is electrically shifted and tilted so that deflection aberration can be removed.
A compensation deflection electromagnetic field is given by the compensation deflection coil 8 and the compensation deflection electrode 9 so that the deflected orbit 3 concurs with the optical axis of the objective lens 7, and the objective lens 7 is tilted at the same angle as the deflection angle and is shifted. That is, the compensation deflection coil 8 and the compensation deflection electrode 9 are arranged in the electronic optical system 1 as a means for achieving xe2x80x9ccompensation fieldxe2x80x9d to electrically shift and tilt the electrostatic magnetic field complex objective lens 13 to remove deflection aberration of the deflected primary electron beam for the purpose of performing the observation on a tilted image.
In the electron beam optical system shown in FIG. 5, a second deflector 6 is arranged so that a tilting direction of the primary electron beam with respect to the specimen 4 can be changed.
As described in JP-A-11-67130, the compensation field for compensating abaxial aberration of the tilted primary electron beam can be given by the following expression:
(1/2) rBxe2x80x2+rxe2x80x2B+(1/2) rxcfx86xe2x80x3+rxe2x80x2xcfx86xe2x80x2xe2x80x83xe2x80x83(1)
where B denotes axial magnetic field distribution of the electrostatic magnetic field complex objective lens 13, xcfx86 is an axial electrostatic potential of the electrostatic magnetic field complex objective lens 13, and r is a distance from the optical axis 2 to the orbit of the primary electron beam.
To tilt the primary electron beam and to compensate abaxial aberration, the compensation field to provide axial magnetic field distribution as given in the expression (1) must be superimposed on a converging field of the objective lens. In fact, however, it is difficult to manufacture the deflector, etc. to achieve such complicated axial magnetic field distribution.
It is an object of the present invention to provide a scanning electron microscope system, by which it is possible to compensate abaxial aberration of a tilted primary electron beam without requiring deflectors to make up a complicated compensation field, and it is possible to perform observation on a tilted image with relatively simple structure.
To attain the above object, the present invention provides a scanning electron microscope system with an electrostatic magnetic field complex objective lens, comprising at least two or more deflection means for tilting a primary electron beam and for projecting the primary electron beam onto a specimen, wherein one of the deflection means is arranged near the objective lens so as to generate a deflection field and also to serve as a compensation field for compensating abaxial aberration at the same time, and abaxial aberration of the primary electron beam deflected by the deflection means is compensated. Also, the present invention provides the scanning electron microscope system as described above, wherein either an electrostatic deflector or a magnetic field deflection coil is used as the deflection means for tilting the primary electron beam. Further, the present invention provides the scanning electron microscope system as described above, wherein the system comprises a specimen stage where the specimen is placed and a stage moving mechanism for moving the specimen stage, wherein the primary electron beam is tilted by the deflection means and simultaneously the specimen stage is moved to correspond to an amount of the deflection, and the same point on the specimen can be always observed even when a tilt angle of the primary electron beam is different. Also, the present invention provides the scanning electron microscope system as described above, wherein the system further comprises an arithmetic unit, two or more specimen images by the primary electron beam projected at different tilt angles onto the specimen are acquired, and a 3-dimensional specimen image is obtained by arithmetic processing of the plurality of specimen images by the arithmetic unit. Further, the present invention provides the scanning electron microscope system as described above, wherein the arithmetic unit measures configurations including a distance between two points on the specimen and depth from the single 3-dimensional specimen image obtained by the synthetic procedure.